Stabilization of wood and wood products with acrylic-like compounds



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3,077 417 STABIMZAHUN F WGD ANB WGD PRQD- UCTS WITH ACRYLIC-UKE SMPUNDS Duane L. Kenaga, Midland, Mich., assigner to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Feb. 5, 1958, Ser. No. 713,287 1li Claims. (Cl. IVI- 59) This invention relates to stabilization of wood and `natural and artificially formed lignocellulose containing materials and more particularly to a new and useful method for the stabilization of wood and wood-like products `against dimensional change and to compositions `for ef- `upon absorption of moisture from the atmosphere and a contraction when moisture is given up to the atmosphere.

yIn a very humid atmosphere, swelling continues until a -moisture content of between 28 and 30 percent is reached.

This moisture content is known as the liber-saturation point. Below the fiber-saturation point, the water enters the fine capillary structure of the cell walls and affects the dimensions of the wood. Above this moisture content, the water enters the cell cavities or the coarse capillary structure. The water present in the coarse capillary structure of the wood is called free water and does not affect the swelling and shrinking under normal conditions.

tabilization of wood against dimensional changes has been attempted heretofore with only limited success. Some of the more important general methods which have been employed are (l) coatings, both external and internal; (2) deposition of bulking agents in the cell walls; (3) decreased hygroscopicity plus bulking; and (4) chemically cross-linking the cellulose molecules, thereby reducing the degree to which wood can swell. An example of the cross-linking method of dimensional stabilization is the treatment of wood with formaldehyde. The stabilization is obtained by a reaction between formaldehyde and the hydroxyl groups of the cellulose molecules connecting together the latter by cross-bridges of acetal linkages. One of the principal disadvantages of the formaldehyde treatment is that it requires anhydrous conditions and very corrosive mineral acids as catalysts, resulting in extensive hydrolytic degradation of the wood. An example of the bulking technique for the stabilization of wood is the impregnation of the swollen wood with a thermosetting resin. In this method, an unpolymerized resin is permitted to diffuse into the cell walls and the treated wood heated to polymerize the resin within the cell walls to form an irreversible polymerized compound which holds the wood in a swollen state. This treatment, however, changes the physical properties of wood, notably its resiliency and shock resistance. Furthermore, resintreated wood is very heavy and very hard. The resin process is also disadvantageous from the treatment standpoint in that it is only practicable for veneer or plywood. A more recent theory under which wood is stabilized is by reduction of hygroscopicity plus bulking. The theoretical principle is thought to be a replacement of some of the hydroxyl groups of the cellulose by reaction with the treating material plus bulking the cell walls by the specific volume of the treating material. An example of this technique is acetylation with acetic anhydride and catalysts and swelling agents such as pyridine or dimethylformamide. This procedure is normally not practical for large cross-section treatment.

It is, therefore, an object of this invention to provide a new andnovel method for the dimensional stabilization of wood and other lignocellulosic materials. A further object is to provide a method for reducing the shrinkage and swelling of wood. A still further object is to provide wood resistant to warping and checking. A still further object is to provide a dimensionally stabilized wood which Vis free of the degradation caused by using known crosslinking agents and catalysts. These and other objects will become evident from the following speciiication and claims.

FIGURE 1 represents a schematic representation of the steps set forth in the following specification for achieving the dimensional stabilization of lignocellulosic materials;

FlGURE 2 represents a modified series of steps which are described in the following specification for the dimensional stabilization of lignocellulosic materials wherein the swelling agent and the impregnant are introduced in a single step.

In accordance with the present invention, it has been found that effective dimensional stabilization of any wood and other lignocellulosic materials, can be obtained by treatment thereof with a non-leachable, non-swellable, insolubilizable compound having radiation-activatable reactive groups and irradiating the so-treated wood under conditions whereby a significant proportion of the treating compound is retained in the lignocellulosic structure. The resulting wood product, containing from l0 to 50 percent, by weight, or more, of the compound in the form of a non-leachable, non-swellable, water-.insoluble product associate/d with the wood, is found to be stabilized against dimensional changes, warping and checking resulting from effects of moisture. Although the amount of compound desirable in the wood structure will vary Vwith the intended use of the stabilized material, the presence of l5 to 30 percent by weight of the compound is 'considered preferable for most purposes.

By employing the treatment of the present invention, wood may be dimensicnally stabilized or made resistant to swelling and shrinking without altering greatly its phys- Vical properties.

Furthermore, anhydrous conditions are not required in carrying out this process. The present f treating compounds may be introduced into the wood from substantially any wood-swelling solvent, such as aqueous or organic solvent solutions. The subsequent irradiation provides a Vdegree of stabilization or reduction in swelling of from about l0 to 70 percent or more. The percent reduction in swelling depends in part on the weight of the compounds retained by the wood.

In accordance with this invention, wood is immersed inthe impregnating compound or in the impregnating compound dissolved or dispersed in a wood-swelling agent to assist in the penetration of the former into the 'cell wall and into intimate Contact with the cellulosic structure. Alternatively, the wood may be swelled with a wood swelling agent and then immersed in the impregnant or a solution of the impregnant. This immersion or impregnation step may be conducted at atmospheric pressure or at subor super-atmospheric pressure, and at room temperature or slightly elevated temperatures. Such impregnation methods are well known to those versed in the art of wood preservation. It is generally preferred to operate at a temperature range of about from 15 to 85 C. Such temperatures give good results, although somewhat higher and lower temperatures may be employed without materially effecting the results. The treating compound is commercially preferably applied from aqueous solution, but solutions in organic solvents may also be employed with equal succe'sswhen good recovery permits reuse of the solvents. In some cases, the treating compound may also serve as the swelling agent or solvent. Suitable organic solvents are those which swell Wood, such as "methanol, ethylene 'glycol methyl ether, N-rnethylpyrrolidone, dioxanes, dimethylformamide, diethylene glycol, di'ethyle'ne. glycol'methyl ether, pyridine, n-butyl- 'amine, piperidine','morpholine,4picoline, 2-picoline, di-

ethy1amin`e,"aniline, acetone, .and mixtures thereof, or aqueous solutions or mixtures thereof. v

Various treating compounds have been fournd satisfactory. Some of such compounds are those having the general formula .and

wherein m is an integer from to 4, inclusive. Thus, acrylonitrile, ethyl acrylate, methyl acrylate, acrolein, acrylic acid, N-rnethylacrylamide, alkyl acrylate esters, N,Ndimethylacrylamide, propyl acrylate, butyl acrylate, methacrylonitrile, ethacrylonitrile, m-niethylacrolein, a-ethylacrolein, methacrylamide, ethacrylamide, and

the like may be employed in accordance with the present invention.

The concentration of impregnating or treating cornpound in the solution will depend in part on the specific treating compound, the solvent, the temperature, and the type of irradiation. Generally, a -30 percent solution has been found to be convenient, although any suitable concentration may be employed.

The preferred conditions of treatment will depend on the particular wood or wood product, the dimensions thereof, the treating compound, the type and degree of irradiation, and the solvent and in some instances other factors. After an immersion period in the treating solution, impregnated wood is cured by exposure to ionizing radiation for a suitable period of time to bring about a reaction, bonding or other association between the treatingl compound and the cellulosic material so that a signicant proportion of the treating compound is retained within the wood structure. ln

`selecting a suitable radiation source, high-energy particwlate radiation or high-energy electromagnetic radiation may be employed. Thus, one may employ atomic particles, neutrons, photons, gamma rays, X-rays, electrons, deuterons, and fission fragments from nuclear reactors or accelerators of from artificial or natural radio-active isotopes. Good results have been obtained by irradiation from sources emitting from l02 to 107 rads/sec. such as a 2 rn.e.v. (million electron volts) source at beam currents up to 145 microamps. Good results have also been obtained by irradiation from a cobalt 60 source at dose rates up to 650,000 rads/hr. (698,750 reps/hn). Wood or wood products may be irradiated up to a dose of l0'I rads. Higher doses will probably cause severe degradation of the cellulosic structure.

After the curing step, the unassociated treating agent and swelling agent may be removed from the wood. The result of this series of operations is that a significant residue of stabilizing compound is reacted with or in some manner associated with and retained in the wood structure, imparting thereto the property of resistance of dimensional change resulting from variations in ambient humidity. l

, in one procedure for treating wood to obtain dimensional stabilization, an air-dried ponderosa pine sapwood block, for example, is weighed and accurately measured in the tangential direction.` The wood vis then impregnated withA one of the treating compounds by vacuum impregnation at an appropriate temperature and pressure. In many instances, the impregnate is a solution of the compound dissolved or dispersed in a woodswelling agent, such as dimethylformamide. Subsequently, the block is removed from the solution and may be sealed in a waterand air-impervious, plastic material to prevent loss of treating solution. The so- -packaged Wood is irradiated to from 102 to 107 rads by a 2-million-volt-electron beam with a current of 5.6 to 56 microarnps. with the wood in the electron beam about 20 percent of the time. In other instances, the impregnated wood is irradiated in a cobalt 60 source at a dose rate of about 156,000 rads per hour. Following the irradiation, the Wood, if sealed in the bag, is removed from the plastic bag and conditioned in air to remove unreacted volatile solvents and/or unreacted or solventsoluble compounds, and leached with running distilled water until the pH of the leached water is the same as that of the untreated concurrently leached controls to remove any unassociated compound or water-soluble solvents.

For the purposes of comparison and to determine the degree of dimension stability, the leached wood pieces are oven-dried at 1D0-105 C., measured along the tangential direction, re-weighed, soaked in water, and remeasured. From the difference in the dired weights before and after treatment with the compounds, the percent compound in the treated Wood can be determined. From the difference in tangential dimensions of the treated soaked wood and the treated oven-dried wood, the percent swelling of the treated wood can be determined. Similar measurements are made on untreated wood. From the data, the degree of dimensional stabilization or reduction in swelling can be calculated according to the following equation:

Percent reduction in swelling Percent swelling of (Percent swelling of untreated wood treated Wood) Percent swelling of untreated Wood The following examples are illustrative of the present invention, but are not to be construed as limiting the same.

Example 1 Clear ponderosa pine sapwood wafers or blocks measuring 1.375 inches radially by 2 inches tangentially by 0.25 inch longitudinally were weighed and accurately measured along the tangential direction. Certain of these wafers were immersed in methanol containing 25 percent acrylonitrile, by subjecting the wood blocks iirst to a reduced pressure of 3-4 mm. of mercury for 15 minutes and then introducing into the evacuated chamber the methanol-acrylonitrile solution to submerge the blocks, breaking or compounds. The samples were then leached in running water for three days. The blocks were then carefully dried to below liber-saturation point to prevent checking and oven-dried at 105 C. for 24 hours. The wafers were then measured and weighed at the oven-dried condiwherein n represents an independent integer from to 2, inclusive, and R represents a radical selected from the group consisting of N-Me Pyrrolidone.. Dirrithyltormamidc. o

do 25 percent rrethacrylami c tions to determine retention and the tangential dimen- Hgh-.o sions, and then were reswelled in water to obtain the CIN swollen dimensions. The acrylonitrile retained in the wood was shown by a weight gain of the Wafer of 28.8 o` percent. The measured percent reduction in swelling cal- (1% 0 (CHmH culated in accordance with the formulahereinbefore presented was 40.2 percent. E /GHDH Example 2- "'N., Clear ponderosa pine sapwood wafers or blocks meas- (CHQ'PH uring 1.375 inches radially by 2 inches tangentially by 15 and 0.25 inch longitudinally were weighed and accurately measured along the tangential direction. Certain of these ii -C-O-CH,CH CH wafers were immersed in dioxane containing percent 0f acrylonitrile by subjecting the W00d b lOCkS IS t0 a wherein each symbol m represents an independent integer Vacuum 0f 3-4 mrIl- 0f mercury for 15 mllllleS and then 20 from 0 to 4, inclusive; and, irradiating the impregnated introducing into the evacuated chamber the dioXane-acrymaterial with from 102 to 10i fads of high energy radialonitrile solution to immerse the blocks. After breaking tion foifrom 1 see, to 24 hours, the vacuum and holding the blocks immersed for l0 min- 2. A process vas set forth in claim 1 wherein the Said utes, they were removed and packaged in small plastic acrylic compeundis aci-elem, Polyethylene bags t0 Prevent 1053 0f dlOXall@ an@ aCfYlO- 25 3. A process as set forth in claim 1 wherein the acrylic nitrile. The so-packaged wafers were then subjected to `eompoui-ld is acrylonitrile, ionizing radiation from a Van de Graai machine to a 4. A process as set forth in claim 1 wherein the acrylic dose of l0I rads. Following the irradiation, the wafers compound is acrylic acid. Wre IemOVed from the Polyethylene bags, CODdlOHed 111 5. A process as set forth in claim l wherein the acrylic a hood to remove any unreacted volatile solvents, monoeempoimd is methyl aerylate. mers, or compounds and then leached in running distilled 6 A process as set foi-th in claim 1 wherein the acrylic water for three days. The blocks were then carefully compound is acrylamide, dried t0 beiOW fiber-Saturation POII i0 Prevent Checking 7. A process for the dimensional stabilization of wood and oven-dried at 105 C. for 24 hours. The wafers Were which comprises the steps `of swelling the wood with an then measured and weighed at the oven-dried c0ndit10ns 35 organic swelling agent while simultaneously impregnatto determine retention and the tangential dimensions, and ing the wood with an acrylic compound having the forthen were reswelled in water to obtain the swollen dimenmula sions. The acrylonitrile retained in the Wood was shown (CHZMH by a weight gain of the wafer of 30.2 percent. The measured percent reduction in swelling in accordance with the H2O: "R formula shown hereinbefore was 34.3 percent. wherein n represents an independent integer from 0 to 2, Examples 3 20 inclusive, and R represents a radical selected from the group consisting Of In the manner of the foregoing examples, employing I ponderosa pine sapwood blocks, and with substitution of HC=0 various swelling agents and treating agents, thel following results were obtained: "CSN Type and Dose of Irradiation in Percent Percent Example Swelling Agent Treating Agent und Amount in Rads: E-Flectron Redue- W eight No. Percent by Weight (Van de Graad), tion in Increase -Ganrra Swelling (Cobalt Ditrethylformamidc.. 2O percent ethyl acrylate E 4155x106 16.5 19.1 do do E o.93 iof 25.8 28.0 MeOH 26 percent methyl acrylate E 107 25.1 40. O None. Acroleln 41. 6 18. 4 MeOH 25 percent acrylic ae 10i 15. 3 3. 0 Meoii E 101.. 29.6 24.6 Dimetliylormarndc. 22 percent acrylic ac E 107 18. 9 11. 5 MeOH 25 percent methyl acrylic a E 10 20. 8 24v 6 McOH 25 percent methyl acrylate.. E 101 27.9 40.1 Dimethylformamide- 20 percent acrylonitrile 17. 6 21.9 Dioxane 25 percent acrylonitrile.. 34. 3 30. 2 do do 29.3 21.6 Ethylene glycol 37. 6 28. 7

methyl ether.

Allylacrylate -C-O (CHQLBH (CH2) mH ehr/7,417

wherein each symbol m represents an independent integer from 0 to 4, inclusive; and, irradiating the impregnated material with from 102 to 107 rads of high energy radiation for from 1 second to 24 hours.

8. A process for the dimensional stabilization of wood which comprises the steps of impregnating the wood with an acrolein and irradiating the impregnated material with from 102 to 107 rads of high energy radiation for from 1 second to 24 hours.

9. A process for the dimensional stabilization of wood which comprises the steps of impregnating the Wood with an acrylonitrile and irradiating the impregnated material with from 102 to 10'1 rads of high energy radiation for from 1 second to 24 hours.

10. A process for the dimensional stabilization of wood which comprises the steps of impregnating the wood with an acrylamide and irradiating the impregnated material with from 102 `to 10" rads of high energy radiation for from 1 second to 24 hours.

11. A process for the dimensional stabilization of wood which comprises the steps of impregnating the Wood with allyl acrylate and irradiating the impregnated material With from 102 to 107 rads of high energy radiation for from 1 second to 24 hours.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES BNL 389 (T-73), Progress Report on Fission Prod-` ucts Utilization VIL Brookhaven National Lab., 1946,

esp. p. 19.

Sun: Modern Plastics, vol. 32, No. 1, September 1954, pp. 141-144, 146-148, 150, 229-233 and 236-238.- 

1. A PROCESS FOR THE DIMENSIONAL STABILIZATION OF WOOD WHICH COMPRISES THE STEPS OF SWELLING THE WOOD WITH AN ORGANIC SWELLING AGENT, IMPREGNATING THE WOOD WITH AN ACRYLIC COMPOUND HAVING THE FORMULA 